Warn the user not to call their user "mock"
Warn the user not to call their user "mock"

Added some text pointing at "mock" install instructions.

Using "<user>" "mock" gives a confusing error message:
```
[root@st30 ~]# useradd mock -G mock
useradd: group mock exists - if you want to add this user to that group, use -g.
```
djs55/planex,simonjbeaumont/planex,simonjbeaumont/planex,djs55/planex,simonjbeaumont/planex,djs55/planex